Fixed several issues related to not met assertions.
See below in the per file description.

Test: fast/css/counters/element-removal-crash.xhtml

* dom/ContainerNode.cpp:
(WebCore::ContainerNode::removeChildren):
Fixed the fact that Node::detach() used to be called
while the DOM tree was in an inconsistent state.
* rendering/RenderCounter.cpp:
(WebCore::RenderCounter::rendererRemovedFromTree):
Introduced this function to remove counters from
descendents of renderers removed from the renderer
tree not only from the removed renderers themselves.
* rendering/RenderCounter.h:
* rendering/RenderObjectChildList.cpp:
(WebCore::RenderObjectChildList::removeChildNode):
Changed to call RenderCounter::rendererRemovedFromTree
instead of RenderCounter::destroyCounters.
2011-05-08 Abhishek Arya <inferno@chromium.org> and Carol Szabo <carol@webkit.org>